In a wavelength division multiplexing (WDM) optical transmission system, optical signals at a plurality of wavelengths are encoded with digital streams of information. These encoded optical signals, or optical channels, are combined together and transmitted through a series of spans of an optical fiber comprising a transmission link of a WDM fiberoptic network. At a receiver end of the transmission link, the optical channels are separated, whereby each optical channel can be detected by an optical receiver.
While propagating through an optical fiber, light tends to lose power. Yet some minimal level of optical channel power is required at the receiver end to decode information that has been encoded in an optical channel at the transmitter end. To boost optical signals propagating in an optical fiber, optical amplifiers are deployed at multiple locations, known as nodes, along the transmission link. The optical amplifiers extend the maximum possible length of the link, in some instances, from a few hundred kilometers to several thousand kilometers, by amplifying optical signals to power levels close to the original levels of optical power at the transmitter end.
An erbium-doped fiber amplifier (EDFA) is one of the most practical types of optical amplifiers employed in many modern fiberoptic networks. A single EDFA module can amplify up to about a hundred of optical channels at a time, thus providing significant cost savings. One of the main components of an EDFA is a length of an active optical fiber having a core doped with ions of a rare earth element such as erbium. In operation, the erbium doped fiber (EDF) is optically pumped by using a suitable pump such as a laser diode, so as to create a population inversion between energy states of the erbium ions comprising a gain medium of the EDF. Referring to FIG. 1, an energy diagram 10 of an erbium ion is presented. The pump light at 980 nm is used to excite the erbium ion from the ground state 4I15/2 into the excited state 4I11/2. A transition to the state 4I13/2 occurs spontaneously with a time constant τ of 5-10 us. As a result, a population inversion between the states 4I13/2 and 4I15/2 is created.
Once the population inversion occurs, the gain medium begins to amplify an optical signal having a wavelength of approximately 1550 nm+/−20 nm propagating along the core of the EDF. The optical signal comprises a plurality of individual optical channels. The gain medium is characterized by a wavelength-dependent gain coefficient, from which amplification coefficients of these optical channels can be determined. During the amplification process, the optical power of the pump is absorbed by the gain medium, which simultaneously amplifies all the optical channels present. The amplification coefficient of a particular channel depends on the optical power and the number of optical channels present, and on the optical power of the pump light. When the number of optical channels changes suddenly, for example, due to adding, dropping, or routing of some of the optical channels, the gain coefficient of the gain medium of the EDF changes as well, which impacts the amplification coefficient of the rest of the optical channels. This phenomenon is highly detrimental because it affects reliability and the bit error rate (BER) of fiberoptic communications links.
To overcome the gain sensitivity to optical power of the signal, various techniques of gain stabilization of an optical amplifier have been developed. At least two types of such techniques exist to date. The techniques of the first type involve detecting input and, or output optical power levels of an optical amplifier and using an electronic feedback loop to adjust the optical pumping levels in the optical amplifier, so as to compensate for optical gain variation.
Turning to FIG. 2, a block diagram of a prior-art optical amplifier 20 of the first type having a gain stabilization circuit 21 is shown. An optical signal coupled to an input port 22 of the optical amplifier 20 is split using a 5% optical tap 23, and the tapped signal is directed to a photodiode 24 for measuring input optical power level, which is used to generate a so called “feed-forward” control signal. A wavelength division multiplexor (WDM) 25 is used to couple light emitted by a pump laser diode 26, together with the optical signal, into an EDF 27, which amplifies the signal as explained above. The output signal is tapped off by an output 2% tap 28 for measuring the output optical power level using a photodiode 29, which is used to generate a so-called “feedback” control signal. Then, the amplified optical signal is directed towards an output port 30. The gain stabilization circuit 21 adjusts the driving current of the pump laser diode 26 based on the feed-forward and feedback signals, so as to stabilize the overall gain of the optical amplifier 20.
Unfortunately, the prior-art gain-stabilized optical amplifier 20 suffers from transient gain fluctuation effects. Referring now to FIG. 3, a time dependence 31 of optical gain of the prior-art optical amplifier 20 of FIG. 2 is shown. When input optical power level changes abruptly as shown at 32, and the optical pumping level is adjusted as shown at 33, the optical gain 31 undergoes an overshoot 34 and an undershoot 35. One fundamental reason of existence of overshoot 34 and the undershoot 35 in the time dependence 31 of the optical gain is a finite transition time τ of 5-10 μs between erbium energy levels 4I11/2 and 4I13/2 shown in FIG. 1. Due to the finite transition time τ, the population inversion between the levels 4I13/2 and 4I15/2 does not changes instantly, even when the optical pumping level 33 is changed very quickly after the input power change 32 is detected. Thus, the techniques of first type generally have an inherent drawback of exhibiting transient gain variation, which is difficult to suppress using feed-forward and/or feedback control signals.
The techniques of the second type attempt to directly stabilize the gain of the amplifying medium. These techniques are commonly referred to as “gain clamping”. One well-known prior-art method of gain clamping is to create a lasing cavity in an optical amplifier. In an article entitled “Gain-Clamped Fiber Amplifier with a Short Length of Preamplification Fiber” by Kyo Inoue, IEEE Photonics Technology Letters, v. 11, No. 9, 1999, which is incorporated herein by reference, an optical amplifier gain-clamped by a ring laser cavity is described. The gain is stabilized because the roundtrip optical losses of a continuously emitting laser equal the roundtrip gain in the amplifying medium. When the losses stay constant, the gain stays constant and therefore the population inversion stays constant. Unfortunately, the transient effects are still present in laser-based gain stabilized amplifiers because of transient effects in the lasing cavity itself. The transient effects in the lasing cavity are observed upon an abrupt change of the input optical power. Furthermore, lasing-based gain stabilization techniques generally suffer from a drawback of increased optical noise in the output optical signal.
In U.S. Pat. No. 7,511,881 entitled “Amplified Spontaneous Emission Reflector-Based Gain-Clamped Fiber Amplifier” by Ahn et al., which is incorporated herein by reference, a gain-clamped optical fiber amplifier is described. The gain of the optical amplifier of Ahn et al. is stabilized by using an optical interleaver coupled to a mirror for reflecting amplified spontaneous emission (ASE) back into the amplifier. The optical power of ASE decreases when the input signal power increases, and vice versa, so that when ASE is reflected back into the amplification medium, the gain is stabilized. Disadvantageously, about a half of the usable amplifier bandwidth is lost due to having to spectrally separate ASE from the signal using the optical interleaver.
The prior art, therefore, is lacking a practical, full-bandwidth, transient-suppressed, gain-stabilized optical amplifier. Accordingly, it is a goal of the present invention to provide such an optical amplifier; in particular, an amplifier having reduced variation of the optical gain with the signal power.